/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.tomcat.dbcp.pool2;

/**
 * An interface defining life-cycle methods for instances to be served by an {@link ObjectPool}.
 * <p>
 * By contract, when an {@link ObjectPool} delegates to a {@link PooledObjectFactory},
 * </p>
 * <ol>
 * <li>
 * {@link #makeObject} is called whenever a new instance is needed.
 * </li>
 * <li>
 * {@link #activateObject} is invoked on every instance that has been {@link #passivateObject
 * passivated} before it is {@link ObjectPool#borrowObject borrowed} from the pool.
 * </li>
 * <li>
 * {@link #validateObject} may be invoked on {@link #activateObject activated} instances to make
 * sure they can be {@link ObjectPool#borrowObject borrowed} from the pool. {@link #validateObject}
 * may also be used to test an instance being {@link ObjectPool#returnObject returned} to the pool
 * before it is {@link #passivateObject passivated}. It will only be invoked on an activated
 * instance.
 * </li>
 * <li>
 * {@link #passivateObject} is invoked on every instance when it is returned to the pool.
 * </li>
 * <li>
 * {@link #destroyObject} is invoked on every instance when it is being "dropped" from the pool
 * (whether due to the response from {@link #validateObject}, or for reasons specific to the pool
 * implementation.) There is no guarantee that the instance being destroyed will be considered
 * active, passive or in a generally consistent state.
 * </li>
 * </ol>
 * {@link PooledObjectFactory} must be thread-safe. The only promise an {@link ObjectPool} makes is
 * that the same instance of an object will not be passed to more than one method of a
 * <code>PoolableObjectFactory</code> at a time.
 * <p>
 * While clients of a {@link KeyedObjectPool} borrow and return instances of the underlying value
 * type {@code V}, the factory methods act on instances of {@link PooledObject
 * PooledObject&lt;V&gt;}.  These are the object wrappers that pools use to track and maintain state
 * information about the objects that they manage.
 * </p>
 *
 * @param <T> Type of element managed in this factory.
 * @see ObjectPool
 * @since 2.0
 */
public interface PooledObjectFactory<T> {

    /**
     * Creates an instance that can be served by the pool and wrap it in a {@link PooledObject} to be
     * managed by the pool.
     *
     * @return a {@code PooledObject} wrapping an instance that can be served by the pool
     * @throws Exception if there is a problem creating a new instance, this will be propagated to the
     * code requesting an object.
     */
    PooledObject<T> makeObject() throws Exception;

    /**
     * Destroys an instance no longer needed by the pool.
     * <p>
     * It is important for implementations of this method to be aware that there is no guarantee about
     * what state <code>obj</code> will be in and the implementation should be prepared to handle
     * unexpected errors.
     * </p>
     * <p>
     * Also, an implementation must take in to consideration that instances lost to the garbage
     * collector may never be destroyed.
     * </p>
     *
     * @param p a {@code PooledObject} wrapping the instance to be destroyed
     * @throws Exception should be avoided as it may be swallowed by the pool implementation.
     * @see #validateObject
     * @see ObjectPool#invalidateObject
     */
    void destroyObject(PooledObject<T> p) throws Exception;

    /**
     * Ensures that the instance is safe to be returned by the pool.
     *
     * @param p a {@code PooledObject} wrapping the instance to be validated
     * @return <code>false</code> if <code>obj</code> is not valid and should
     * be dropped from the pool, <code>true</code> otherwise.
     */
    boolean validateObject(PooledObject<T> p);

    /**
     * Reinitializes an instance to be returned by the pool.
     *
     * @param p a {@code PooledObject} wrapping the instance to be activated
     * @throws Exception if there is a problem activating <code>obj</code>, this exception may be
     * swallowed by the pool.
     * @see #destroyObject
     */
    void activateObject(PooledObject<T> p) throws Exception;

    /**
     * Uninitializes an instance to be returned to the idle object pool.
     *
     * @param p a {@code PooledObject} wrapping the instance to be passivated
     * @throws Exception if there is a problem passivating <code>obj</code>, this exception may be
     * swallowed by the pool.
     * @see #destroyObject
     */
    void passivateObject(PooledObject<T> p) throws Exception;
}
